This invention relates to the sequential hydraulic fracturing of subterranean formations and more particularly to the forming of a vertical hydraulic fracture in a subterranean formation that is normally disposed to form a horizontal hydraulic fracture.
In the completion of wells drilled into the earth, a string of casing is normally run into the well and a cement slurry is flowed into the annulus between the casing string and the wall of the well. The cement slurry is allowed to set and form a cement sheath which bonds the string of casing to the wall of the well. Perforations are provided through the casing and cement sheath adjacent the subsurface formation. Fluids, such as oil or gas, are produced through these perforations into the well.
Hydraulic fracturing is widely practiced to increase the production rate from such wells. Fracturing treatments are usually performed soon after the formation interval to be produced is completed, that is, soon after fluid communication between the well and the reservoir interval is established. Wells are also sometimes fractured for the purpose of stimulating production after significant depletion of the reservoir.
Hydraulic fracturing techniques involve injecting a fracturing fluid down a well and into contact with the subterranean formation to be fractured. Sufficiently high pressure is applied to the fracturing fluid to initiate and propagate a fracture into the subterranean formation. Proppant materials are generally entrained in the fracturing fluid and are deposited in the fracture to maintain the fracture open.
Several such hydraulic fracturing methods are disclosed in U.S. Pat. Nos. 3,965,982; 4,067,389; 4,378,845; 4,515,214; and 4,549,608 for example. It is generally accepted that the in-situ stresses in the formation at the time of such hydraulic fracturing generally favor the formation of vertical fractures in preference to horizontal fractures at depths greater than about 2000 to 3000 ft. while at shallower depths such in-situ stresses can favor the formation of horizontal fractures in preference to vertical fractures.
For oil or gas reservoirs found at such shallow depths, significant oil or gas production stimulation could be realized if such reservoir were vertically fractured. For example, steam stimulation of certain heavy oil sands would be enhanced and productivity would be optimized in highly stratified reservoirs with low vertical permeability. Creation of such vertical fractures has been disclosed in U.S. Pat. Nos. 4,687,061 and 4,714,115 to Duane C. Uhri. Both these patents disclose sequential hydraulic fracturing techniques for forming the vertical fracture. In U.S. Pat. No. 4,687,061, a subsurface formation surrounding a deviated borehole and having original in-situ stresses that favor the propagation of a vertical fracture is penetrated by a cased borehole. The casing is perforated at a pair of spaced-apart intervals to form a pair of sets of perforations. Fracturing fluid is initially pumped down said cased borehole and out one of said sets of perforations to form a first fracture that is oriented in a direction perpendicular to the direction of the least principal in-situ horizontal stress. The propagation of this first vertical fracture changes the in-situ stresses so as to favor the propagation of a second vertical fracture. This is oriented in a direction perpendicular to the direction of the altered local least principal in-situ horizontal stress. Thereafter, while maintaining pressure in the first vertical fracture, fracturing fluid is pumped down said cased borehole and out of the other of said sets of perforations to form such a second vertical fracture which will now link naturally occurring fractures in the formation to the deviated wellbore.
In U.S. Pat. No. 4,714,115 a subsurface formation having original in-situ stresses that favor the propagation of a horizontal fracture is penetrated by a cased borehole which is perforated at a pair of spaced-apart intervals to form a pair of sets of perforations. Fracturing fluid is initially pumped down said cased borehole and out one of said sets of perforations to form the originally favored horizontal fracture. The propagation of this horizontal fracture changes the in-situ stresses so as to favor the propagation of a vertical fracture. Thereafter, while maintaining pressure on said horizontal fracture, fracturing fluid is pumped down said cased borehole and out of the other of said sets of perforations to form the newly favored vertical fracture.